Silver halide photography depends on the formation of dyes in order to reproduce an image. These dyes are typically formed from couplers present in or adjacent to the light sensitive silver halide emulsion layers which react to image light upon exposure. During development, the latent image recorded by the silver halide emulsion is developed to amplify the image. During this process in which silver halide is reduced to elemental silver, the color developer compound used is at the same time oxidized, as is typical in a redox reaction. The oxidized developer then reacts or couples with the coupler compound present in or adjacent to the emulsion layer to form a dye of the desired color.
Typically, a silver halide emulsion layer containing a magenta dye-forming coupler is sensitized to green light. This facilitates so-called negative-positive processing in which the image is initially captured in a negative format where black is captured as white, white as black, and the colors as there complimentary color (e.g. green as magenta; blue as yellow; and red as cyan). Then the initial image can be optically printed in the correct colors on a reflective background through the device of optical printing which has the effect of producing a negative of the negative or a positive of the image.
Viewable images may also be produced through reversal processing in which the initial negative image is reversed by using a black and white developer, processed to remove the developed silver, and by then fogging the element in the presence of color developer to provide developed silver in proportion to the amount of image light with corresponding dye formation.
One of the difficulties with color couplers is achieving both a desirable dye forming activity and a satisfactory dye light stability. In other words, it is necessary for the coupler to be capable of forming sufficient dye density during the development time of the process (e.g. 90 seconds). It is also desirable that the dye formed by reaction between the oxidized color developer and the coupler exhibit a degree of stability against light degradation which is satisfactory for the type of exposure which the photographic element will be exposed.
Couplers that form magenta dyes upon reaction with oxidized color developing agent are described in such representative patents and publications as: U.S. Pat. Nos. 2,311,082, 2,343,703, 2,369,489, 2,600,788, 2,908,573, 3,062,653, 3,152,896, 3,519,429, 3,758,309, 4,540,654, and "Farbkupplereine Literature Ubersicht," published in Agfa Mitteilungen, Band III, pp. 126-156 (1961). Typically, such couplers are pyrazolones, pyrazolotriazoles, or pyrazolobenzimidazoles that form magenta dyes upon reaction with oxidized color developing agents. The present invention is concerned with the pyrazolotriazole type of magenta dye-forming coupler. In particular, the coupler of the invention is a 1H-pyrazolo[3,2-c][1,2,4] triazole compound. Such couplers have been found advantageous because the dyes formed from such couplers provide improved spectral absorption curves and therefore give better color rendition. Pyrazolo-based couplers have also been suggested as cyan couplers and sufficient dye light stability would be required for such usage.
In a series of published Japanese patent applications, J05-273715; J05-323531; J05-323532; J05-323533; J05-323534; and J05-323538; various suggestions are presented for suitable 3-position substituents for 1H-pyrazolo[3,2-c][1,2,4] triazole compounds. Among the suggestions are amine or amide substituents bonded to a carbon alpha to the 3-position of the pyrazolotriazole nucleus. None of the proposed couplers is adequate.
It is therefore a problem to be solved to provide a photographic silver halide emulsion layer which will exhibit the desired dye forming ability and will also enable the formation of a dye which has improved light stability.